Heat exchange device and temperature control therefor



March 10, 1953 c. R. ALDEN 2,631,218

HEAT EXCHANGE DEVICE AND TEMPERATURE CONTROL THEREFOR Filed sept. 15, 1949 2 SHEETS-SHEET 1 HG1 U) f3@ x39 .f l

5 /Z /5 ZZ 25 20 i?" TIME /N SECONDS IN V EN TOR.

'Caro/ R. Alden mm, WMM/,9a i155- C. R. ALDEN March 1o, 1953 HEAT EXCHANGE DEVICE AND TEMPERATURE CONTROL THEREFOR 2 SHEETS-SHEET 2 Filed Sept. l5, 1949 Patented Mar. 10, 1953 HEAT EXCHANGE DEVICE AND TEMPERA- TURE CONTROL THEREFOR Carroll R. Alden, Detroit, Mich., assignor to Ex-Cell-O Corporation, Detroit, Mich., a corporation of Michigan Application September 15, 1949, Serial N o. 115,941

22 Claims.

The present invention relates generally to means for applying heat -to a uid and more specifically to a system including a unit for raising the temperature of -a moving fluid to a relatively constant value through the application of heat at a controlled rate. The invention finds particular but by no means exclusive utility in connection with the continuous-flow ypasteurization of milk.

Temperature control systems heretofore commercially available for the purposes set forth above gener-ally comprise one or more thermostatic devices arranged to -control a suitable heating means. Such systems operate on the basis of adding heat at the maximum available rate as long as the temperature off the substance being heated remains below a predetermined value. Conversely, these systems immediately reduce the rate of heat application to zero as soon as the temperature of the heated substance exceeds the predetermined value. Such systems consequently tend to overshoot and undershoot, causing substantial fluctuations in the desired ltern- -perature Although simple thermostati-c devices now commercially available are capable of rel-- ably responding to changes in temperature as slight as from 0.1 to 0.01 of 1 F., they are surbject to the severe limitation of indicating a temperature 'deviation qualitatively but not quantitatively. Hence it is -a comm-on practice to utilize a plurality of thermostatic devices, one being adjusted for response to a maximum temperature and `the other a minimum temperature, the optimum or predetermined temperature value being located between the maximum and minimum. Systems using thermostatic devices in this manner have an overall accuracy far Ibelow the inherent accuracy of response of the thermostatic devices.

Accordingly, one object of the present invention is to provide a novel temperature control system lfor heating a moving fluid to a predetermined temperature, such system having an overall accuracy commensurate with the inherent accuracy of response of its thermostatic device.

Another object of the invention is to provide a novel system of the foregoing type which will be ladapted to maintain the temperature of the heated fluid with a high idegree of precision notwithstanding variations in flow as well as in the initial` temperature land composition of the incoming fluid.

. A further object is to provide a novel heater for use in a system ofthe character set forth.

Another object is to provide a temperature control system and heater of the above type and which will be adapted for stable operation with I electrolytic fluids having negative temperature coeiicents of specific resistance.

A more specific object is to provide a temperature control system and heater of the above character and which will be :particularly well adapted yfor the continuous-How pasteurization of fluids such as milk.

Other objects and `advantages will become apparent as the following detailed description f proceeds, taken in connection with the accompanying drawings, wherein:

Figure l is a diagrammatic View of a novel temperature control system and heater embodying the present invention.

Fig. 2 is a transverse sectional view 1taken horizontally through the heater illustrated in Fig. 1 and in the plane of the line 2 2.

Figs. 3, 4 and 5 are theoretical temperature curves illustrating diagrammatically the =per formance of the system of Fig. 1 under various operating conditions.

Fig. 6 is a diagrammatic view of a modied temperature control system also embodying the invention and adapted for use with a continuousflow pasteurizer for iiuids such as milk.

Fig. 7 is a diagrammatic view of another modified system also embodying the invention.

While the invention is susceptible of various modifications and alternative constructions, the preferred embodiments have :been shown in the drawings and will be described below yin considerable detail. It should be understood, =how-- ever, that there is no intention to limit the invention to the specific forms disclosed, but on the contrary, the invention is to cover all modifications and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims.

Referring more specifically to Figs. 1 and 2,

there is shown an illustrative temperature control system I for raising to a predetermined level the temperature of a continuously flowing stream of fluid. In the :present instance. the system lI0 includes an electrical heater i l which is designed for the continuous-flow pasteurization of an electrolytic fluid such as milk. The heater Il raises the temperature of the fluid by passing an alternating current between spaced electrodes l2 immersed in, or in contact with, the iiuid stream. Power may be supplied to the system i0 and heater Il from any suitable source of alternating current via main line switch I4 and line Iconductors Li, L2. Although the invention is equally well adapted for use with heating devices `energized from multiphase sources, a single-phase heater has been shown for purposes of simplicity in illustration.

Turning rst to the heater` H, it will be perceived that the latter comprises a hollow body l5 havingwalls defining therein a uid passage or heating chamber i t running longitudinally of the body. At its lower end, the body l5 is provided with a fluid inlet i8 and at its upper end; with a iiuid outlet l 9, both the inlet and the outa let communicating freely with the chamber i6. The inlet i8 and outlet i9y may, of: course, be connected in any Well-known manner to external conduits adapted to direct a flow of iiuid through the chamber le for processing. Whilethe invention is not concerned with the assembly details of the body l5, it might be noted in passing that, the inlet and outlet fittings are secured to the central or electrode section of the body by means of nonconductive bands Ztl, an appropriate gasket, 2l being interposed between each tting and the adjacent edges of the electrodes i2.

Provision is made in the system le for controlling within narrow limits the iinal temperature of the iiuid passing through the heater Il by utilizing to the fullest possible extent the inherent sensitivity of a single throw temperature responsive or thermostatic device which normally reacts to temperature changes at a predetermined level. This isaccoinplished by causing thesystem to effect continuously and automatically changes in the rate of heat application, each 'such change being of appropriate direction andof av magnitude which is proportional to the need for the change. Accordingly, the system Il) includes a principal heat. control means and a secondary heatV control means both adapted to alter. the heat input to the luid in thel heater. The principal heat control means is adapted to effect a relatively slow, progressive and unlimited response to a departure of the iinal fluid temperature from itspredetermined value. The second'ary heat control means, on the other hand, is adapted to effect an immediate, total and limitedrespons'e to a departure of the nal iuid temperaturey fromy the predetermined value. Both the principal and the secondary` heat control means 'are governed by the thermostatic device` in a manner which renders the latter responsive tothe difference between the average temperature of the fluid leaving the heater and the predetermined temperature for which the thermostatic device has been set. When this temperaturediierence is slight, the rate of change of ifeatapplication is slight. Conversely, when this temperature difference is great, the rate of change of heat application is made appropriate both in direction and in magnitude to bring the average temperature of the fluid leaving the heateras close as possible to that temperature.

fonwhi'ch the thermostat is adjusted, the rate of" change automatically decreasing as the. predetermined temperature is approached.

Referring once more to Figs. 1 and 2, there is schematically shown athermostatic device 22 projectinginto the heating chamber le adjacent the outlet l. The device Z2 is preferably a simplebimetallic thermal switch having a pair of contacts 2li-With their adjustment centered on the predetermined temperature at which it is desiredl to maintain the fluid leaving theheater H;- iny the'present instance, the contacts 2`4are 4 adapted to close when the nal or outgoing fluid temperature drops below the predetermined value and to open when the outgoing fluid temperature rises above such value. For all practical purposes, the response of the contacts 24 is substantially instantaneous.

rlihe principal heat control means utilized in the system l il involves an arrangement for varying the input of heat to the iiuid within the heating chamber ES through variation of the eiective electrical resistance interposed in a heating circuit. This is accomplished in the present instance by providing the heater i i with electrodes l2 and a dielectric or nonconductive member adjustably positiomabler to vary the elfective resistance of the volume of liquid between the electrodesy 12;. Thus there is mounted within the heating chamber a nonconductive shield 25 having an integral supporting stem 26 journaled in boss 250i top body panel 2 9. With the foregoing structure, the effective resistance between# the electrodes l may be varied simply luy-alteringV the angular position of the shield 25 so as tochange the cross-sectional area of the electrolytic current path between the electrodes. When positioned with its plane perpendicular to the plane.V

of the electrodes i2, the shield-2h. creates a minimum of resistance to current` flow through the electrolyte within the heating chamber I6. On the other hand, whenrotated190D-fromy this lposition so that its plane becomes parallel tothe plane or the electrodes, thev shieldl 25 createsav maximum of resistance to current iioW, thereby` decreasing substantially theY heat input to theluid within the chamber IE. In order tosmooth out the curve of resistanceY chang-es for-variousangular positions of the shield 25, the latter-isKK preferably fashioned with a substantially diamond shaped cross section.

Angular movement of the shield 25 in either` direction may readily bev effected by means such as reversible electric motor Sil-having a Worm 3| fixed to its drive shaft 32.- Power is transmitted trol means by the thermostatic device`22, there-- is provided a control relay CR having an Vac tuating coil 35 connected in seriesj withl the contactsd across line conductors Ll, L2. By-meansV of an appropriate linkage (notdetailed) lthe con-v trol relay CR is enabled to actuate a motor revers-- ing switch 36 which receives power from-line con..

ductors Ll, L2v via conductors 38, 39. When theV contacts 2li of the thermostatic device are closed due to a drop in iinal fluid temperature below the predetermined level, the relay CR becomesenergized and through the reversing switch 36 causes the motor to drive the shield 25' in--a direction which decreases the efeotive resistancebetweenthe electrodes I2y so as to increase the. energy input to the Iiuid within the heating chamber.-

Conversely, when the contacts 24 open due-toa'n' increase in fluid temperature, the relay CR becomes de -energized and shifts the motor reversing switch into its alternative position wherein the motor roatates the shieldv 25 in theopposite direction so as to increase the resistance between the electrodes and therebyy decrease the energy input to the fluid in theheating chamben Turning now to rrthe secondary heatlcontrol, 1t will be noted thatthe latter comprises-means `for effec-ting slight` changesA` the input offbeat to the fluid within the heating chamber I6. This is accomplished in the present instance by effecting comparatively slight changes in the potential applied to the electrodes I2 by selectively interposing or shunting out of their circuit a relatively small resistance or reactance. Referring to Fig. l, it will be erceived that there is seriesconnected in line conductor L2 a resistor 40 having a value which is sufiicient to create a relatively small drop in the potential applied to the electrodes. This potential drop is desirably only a very small fraction of the potential applied to the electrodes. Its magnitude should, however, be suicient to compensate for changes in the resistance of the electrolytic fluid resulting from slight changes in temperature. The magnitude of the potential drop should also be sufficient to compensate for relatively slight changes in the temperature of the incoming iiuid.

To permit governing cf the secondary heat control means by the thermostatic device 22, there is associated with the resistor 40 a by-pass switch having xed contacts 4l and a bridging contact member 42. The operation of the switch is directly accomplished by the control relay CR. The relationship between these members is such that when the relay CR becomes energized as a result of a drop in fluid temperature and closure of the thermal switch contacts 24, the contacts 4l, 42 of the by-pass switch will be closed and thereby render the resistor 40 ineffective with respect to the electrode circuit. On the other hand, when the control relay CR becomes deenergized due to an increase in fluid temperature, the contacts 4i, 42 simultaneously open and inject the potential drop of the resistor 4B into the electrode circuit so as to effect a decrease in the energy input to the fluid. It should be borne in mind that the cyclical and fractional variations in nal fluid temperature occasioned by rendering the resistor 40 successively effective and ineffective are of primary importance in achieving satisfactory temperature control but produce no substantial changes in the nal fluid temperature because of their close proximity to the predetermined temperature at which the device 22 respends.

In operation, when the shield 25 has its optimum position for a given rate of ilow and initial temperature of the incoming fluid, the contacts 24 of the thermostatic device 22 open and close in continuing cycles of relatively short duration at a ratio of open to closed time approximately equal to 1. Such action is shown diagrammatically in Fig. 3 where, for purposes of illustration only, it is assumed that the predetermined temperature level to be maintained in the outgoing fluid is 162 F. and that the duration of one complete operating cycle of the thermal switch con-v tacts 2li is approximately 10 seconds. Under these conditions, the motor 30 will operate for equal times in each direction with the net result of no eective traverse for the shield 25. In other Words, upon opening of the contacts 24 as a result of a fractional excess ln the final fluid temperature over the predetermined level, institution of motor operation in the proper direction to increase the chamber resistance takes place. The motor will continue to operate in such direction as long as the contacts 24 remain open. When the temperature of the fluid leaving the heating chamber drops fractionally below the predetermined temperature, either because of the change in potential applied to the electrodes by introducing the potentia1 drop of the resistor 40,

or by reason of the change in the position of the" shield 25, or both, the contacts 24 will close and institute motor rotation in a direction so as to decrease the chamber resistance. will continue until the contacts 24 are again opened by the resulting increase in temperature.

If, in any given cycle of minute temperature variations such as those occasioned by the resistor 40, the ratio of open to closed time is greater than unity (as indicated in Fig. 4), the net effect of motor rotation during such cycle will be to alter the position of the shield 25 in the proper direction to increase the effective resistance between the electrodes, or the heating chamber resistance. This reduces the flow of current through the iluid within the heating chamber and thereby reduces the heat energy input thereto. On the other hand, if the ratio of open to closed time is less than unity (as indicated in Fig. 5), the net result of motor rotation will be to decrease the heating chamber resistance and thereby allow more current to flow therethrough so as to increase the heat energy input to the fluid. As long as the ratio of open time to closed time lies between the limits of zero and iniinity, cycling of the contacts 24 will continue. If the ratio of open to closed time becomes innity, cycling will cease and the motor 3B will travel continuously in that direction which increases the chamber resistance so as to eiect a decrease in the heat energy input to the fluid. By the same token, if the ratio of open to closed time becomes zero, cycling of the contacts 24 will also cease and the motor 30 will travel continuously in that direction which will decrease the chamber resistance and thereby increase the heat energy input to the fluid.

In the foregoing system, the rate of heat application subject to the principal heat control means may be rendered adjustable in almost iniinitesimal increments responsive to the ratio O/U wherein O is the elapsed time the nal fluid temperature remains over principal control means in altering the rate of heat application to the iluid in the heating chamber will be zero.

When the ratio O/U becomes infinity, the principal heat control means will alter the rate of heat application in the negative or decreasing direction at the maximum rate of whichA the motor drive is capable, such maximum being represented by the letter M. By the same token,

when the ratio O/U has any Value betweenv unity and innity, the resulting rate ofchange in the application of heat may be expressed as M U/O-1). The value of the parenthetical portion of this expression is always negative and lies between 0 and -1. On the other hand, when the ratio O/U becomes zero, the motor drive is rendered effective to increase the application of heat at the maximum rate M. Where the ratio O/U has any value between zero and unity, the

resulting rate of change in the application of heat to the uid may be expressed by the formulav M(l-O/U). The value of the parenthetical por-1 tion of this formula is always positive underth' This rotation the predetermined value and U is the elapsed time the iinal fluid' 7" foregoing: condition. and. lies.. between. zero; and unity..

TurningmowftorFig. 6, there; is shown anotherI pasteurized. is introduced into. a. heatv exchanger 44 viaza conduit t5... In thevexchanger 44, the incoming fluid picks upvheat from the. outgoing. pasteurizedifluid, whereupon it isled'via a conduit 4.5- to: the) suction sideof a pump 4. The latter forces the iluid through the heater. IIA, where. its4 temperature.- is raised to a predeter-Y mined: value as. determined.- by the setting. of aV thermostatic device-49; The heated fluid leavesthe heater HA via thev device @.9 and. is thence conducted .to a. holding section 5d which is.y of. appropriate. design. to require a predetermined time interval-.for each particle of the fluid to pass therethrough. Upon leaving the' holding section 5.0 thepasteurizedzgiluid enters theot'ner side of. thefheat. exchanger` 44 and thereupon passes to anyoutlet cooler;v 5|- whichis cooled as. by means. of. a` circulating-,water conduit 52.

Heatr energy is' supplied to thev duidl flowing' through-theheater I'IA- as 'by-means of aheating circuit. including' terminals 55 whichy may, for example, be electrodes adapted to pass an electriccurrent therethrough- Power is supplied to the. electrodes. 54 from a" suitable. alternatingy current sourcevialineconductors LLLZ. Althoughan, eleotrolytic.. heating. arrangement has been show-n diagrammatically inthe drawings, it should bebornein mind that other electrically powered .heating4 means could be used with equal facility.

Thexsystem IALlikethe system itl, comprises a principal heat control. means and a secondary heat controlmeans,V both of which are governed by the. thermostatic devicefllwhich is. preferablya thermal .switch having bimetallic contacts 55. In the .systemv IEJA, however, the principal heat control, meanscomprises a saturable-reactor 55. Which-is adaptedto-eiect substantial changes in.`

theenergyinput. to theviluid within .theheater by changing,` the'potential ap-plied to the terminals or. electrodes .54. Accordinglythe reactor 5.5 com-y prisesa mainy winding` 5B. interposed inconductor :2. of i the .heating circuit anda saturatingwinding; 5.91 connected. into asuitablef. direct current con.- trol circuit. The control circuit receives power through'anappropriate rectiiyingmeans i3d connected acrossthe. main power supply LLLZ.

Startingat Vthe. .rectifying means 59, thecontrolcircuitincludes conductor 5i, saturating winding- 59, secondary. heat control means 62, conductor 64 rheostat 65, and conductor 65..

To permit goyerningof-the. principal heat control. means by the thermostatic device. 49, pro.- vision is made for varying the currentthrough. the control circuit, and hence the saturating winding 59, inthe proper direction to olset a de.- parture of Lthe iinal uid temperature from the predetermined value for which the contacts 55 have been-set. Accordingly, there is provided a control relayy CR-I which is series-connected with `the contacts 55 and kupon closure of the same becomes energized from the line conductors'Ll, L2. The relay CR-Iis adapted to vary the current inthe controlcircuit by means ofv reversible motor.: which operates. the rheostat 65. In. thepresentinstance.: the' .motor 6:8; receives .power 8. from. line. conductors.. L l., L2 through a reversing.. switch. 69.1and.a. time kdelay unit, 10'..

.As in the. system l-, the secondary heat.. con.- trol means 62 of the. system. IBA comprises. a. relatively'small. resistor Il and a by-pass switch 'I2 connected in parallel therewith. The thermostatic. device. 49- governs theactuation. of the. secondary heat-control means. 622 through the con.- trol.V relay. CRL-i, thelatter being4 adapted. to actuatethe, by-.pass switch 1.2. Actuation of. the latter,.of course., merely servesto cutthe resistor i t into or shuntitoutof the control circuit, producing a corresponding variation in the. saturaf tion .ofv the. reactor 5.6. with av relatively small change. in the heat energy imparted: to. the fluidv by.l thev electrodes 54..

y In operation, when thetemperature of. thefluid'. leaving. the. heater HA rises. above. the predetermined temperature, the. contacts will. open. This de-energizes the control relay Clit-Iv and immediately opens. the by-pass switch. 12 so as. to. interposethei-ull. value-.of resistor '1.1. into the control. circuit.. Such action consequently reduces. the current in the saturatng Winding 59' and: at the same time causes. acorresponding, slight increase in the potential. dropin. the heating circuitdue. to.. thereactor 56. Such change, decreases the. heat. energyapplied. to the fluid by the` electrodes 54 and tends to. restore the tempenaturev to the.- predetermined level. Conse.-. quently, should. the.A temperature. of` the outgoing. fluid decrease. below such predetermined level., the. contacts 55 will. close andenergize the relay CR-t.. Thisimmediately shunts. out the resistor ll increasing, the current inthe controlcircuit',

mum position for agiven initialand a given. final temperature.` of the. fluid. flowing through. the, beaten. the. contacts. 554 will. cycle. between their. open. and. closed.` positions, the intervals.. ofi open.

time. andvclosedvtime being approximately equal.. Since: the. device lil1 may haveA a. sensitivity as.

sharp as 0.l F., the actual change in tempera..- turefat-.the contacts. 55'may wellhe .nomore than aafraction oia degree.

ingseotion 50., l.this periodic `iluctuationis .reduced to.- anL undetectable. or at least.y an insignificant..

fraction of4 the. temperature. ofy the. fluid leaving.. the holding; section 50.. Where the. deviation. of. thenalfluid temperature from the predetermined .levellis substantial orxtendsto. persist over..

a. considerableperiodiciv time, the oontrolrelay CB14; isadapted toeffect an` adjustment of therheostat 55. through4 the. motor. reversing sw-irsh.`

' ing'. arrangement', each change.- in the. initial tem`- peraturelof the; fluid: will correspondingly aiiect the;r ratio-of opentme to closed time-of the con tacts.` 5.5: Eachsubstantial unbalanoe from. the..

11:1 ratio of! these time intervals. will cause the;v

With the. turbulence and. resulting averaging of temperature. in thehold motor 68 to move the rheostat 65 in the appropriate direction and at a suitable time rate of change to restore the perfectly balanced condition. In Fig. 7, still another embodiment of the invention is shown in the form of a temperature control system HIB having a heater IIB with a fluid inlet 73 and an outlet 14. Heat is applied to the fluid passing through the heater .as by means of an element l which may, for example, be a set of coils or tubes through which an appropriate thermodynamic medium may be circulated. Adjacent the outlet end 14 of the heater, there is mounted a temperature responsive device or thermostat 'IB which is preferably a simple thermal switch having bimetallic contacts ll. The principal heat control means comprises a valve 'I8 actuated by means such as reversible motor 19 so as to control. the amount or" thermodynamic fluid supplied to the main heating elementr'l. The motor 19, in turn, is controllable lthrough a reversing `switch BE! which receives vpower from line conductors LI, L2. The operation of the principal heat control means is, as in the systems described earlier herein, governed by the temperature responsive device I6 through the use of a control relay CR-2 which is arranged to actuate solenoid 82 of the motor reversing `Switch 80.

The secondary heat control means of the system ,IBB comprises a relatively small heating element 85 located within the heater IIB and adapted to have a suitable thermodynamic reedium circulated therethrough. At the inlet end of the element 84 there is provided a suitable 'solenoid operated valve S5 adapted to admit or shut out the thermodynamic medium. The operation of the secondary heat control means is also governed by the temperature responsive device 15, the solenoid 86 of the valve 85 being series-connected to the power source LI, L2 via switch BI of control relay CR-2.

The operation of the system H1B is generally similar to that of the systems already described. Briefly,pwhen the final temperature of the fluid leaving the heater IIB remains substantially at the predetermined level for which the device 'I6 has been set, the contacts TI will alternately open and close for approximately equal periods of time and minor deviations in temperature will be appropriately compensated for by the actuation of solenoid valve 85 in response to operation of the control relay CR-2. When the vaverage iinal temperature departs to the extent that the contacts 'Vl remain open or closed for unequal intervals of time, the motor actuated valve v'H3 will be gradually jcgged to a position where the balance is restored.

I claim as my invention:

l. A temperature control system for raising the temperature of a moving fluid to a predetermined level through the controlled application of heat thereto, said system comprising the combination of a heater having fluid inlet and discharge connections, a single throw thermostatic switch responsive to the nal temperature of the fluid passing through said heater, said single throw thermostatic switch being set to cycle between an open and a closed position about alpredetermined level of final iluid temperature, a principal heat control means for instituting a continuing, progressive and corrective change in the rate of heat application oi said heater. within periods following the response uld temperature departing from said predetermined level, said principal heat control means being governed by said temperature responsive switch, and a secondary heat control means also governed by said temperature responsive switch andv adapted to institute an immediate, total and limited corrective change in the rate of, heat application following response ofsaid tempera,- ture responsive switch to a final fluid tempera'- ture departing from the predetermined level.

2. A temperature control system for raising a moving fluid to a predetermined temperature through the controlled application of heat thereto, said system comprising the combination of a heater, means for passing a stream of iluid through said heater, a single throw thermostatie switch responsive to the temperature of the fluid discharged from said heater, said single throw thermostatic switch being set to cycle between an open and a closed position .about a predetermined level in the temperatureof thef'u'id 'discharged from said heater, al principalheat control means for instituting'a: continuing-and progressive increase in the rate of heat-appli? cation of said heater within periods following the response of said temperature responsive switch to a temperature below a predetermined level and for also instituting a continuing and progressive decrease in the vrate ofheatapplijcation of said heater withinperiods fr )llowing the response ofv said temperature responsive switch to a temperature abovethe predetermined level,- said principal heal; control means beig governed by said temperature responsiveswitch, and a secondary heat control means also-governed by said temperatureresponsive switchand adapted to institute an immediate, total' and limited increase in the rate of heat application of said heater following response of saidtemperature responsive switch to a temperature below the predetermined level and to institute an immediate, total and limited decrease in the rate of heat application of said heater followingresponse 0f said temperature responsive switchV to a temperature above the predetermined level.

3. A temperature control system for increasing the temperature of a moving stream v'of iiuid to a predetermined value through the controlled application of heat thereto, said system comprising the combination of means defining `r'a heating chamber for the iiuid, a single throw thermostatic device centered for operation at said predetermined value and responsive to the iinal temperature of the fluid discharged fromsaid heating chamber, a principal heat control means governed by said thermostatic device and adapted for correctively modifying the rate oi heat energy input to the iluid within said heating chamber vfluid to a predetermined value by the application of heat thereto at controlled rates, said system comprising, in combination, means defining f a heating chamber for the ui-d, a principal heat control means for correctively effecting a gradual and progresslve change in the rate of application heat .to theiiuid within said lhead/ing chamber,

a.. secondary heat control means. for correctively effecting, animmediate, total and limitedchange in the rate of. application. ofi heatV to the. fluid within said. heating chamber.,. and a single throw thermal. switch centered for. operation at said predetermined value and. responsiveto the tem.-

perature ot the f huid. leaving said' heatinf.;h chamber, said thermal. switch being connected with meansfor.- correlating the-operation. of .said prin-.-

cipal.. and. said secondary heat control. means to effect. continuous and. automatic changes. in. the

rata ot heat.. applicationto the fluidwithinv said heating chamben. each such. change, being pro.- portional.. to the difierence between the temperatureof. the fluid. within saidl heating` chamber andthe valueiof.. the, predetermined temperature.

A system. for maintaining aheated fluid. at a. predetermined temperature.- and comprising the combination. o aheater, means for .passing fluid through. said. heater.,y a. single. throw thermal switchseti to. cyclezbetween. open and closedpositons...on.either sidarespeetively of thepredeter.- mined temperatureY and. in. response to. the. tempeinture of. the, i1id.within..said heater,i said switch.` being.y adapted to open immediately when fluid, temperature departsy in. one direction fromthepredetermined. temperature andto close immediately` when the iiuid temperature departs iiithe opposite. direction, asecondary heat. control means forv immediately changing the. rate. of

heat. application by a. relatively small increment 'im either. direction depending upon whether; said switch is= openv or closed, and a principal heat controlf means. for effecting a gradual. progres.- sive. andI corrective net. changey in the.. rate of heat applicationV when the rat-ioofopen-.tima to closed time of-.said switch during a normaly oper.-

` ating.- cyclef dep arts from unity.

6.1. In al system for precisely maintaining the average; temperature of aheated. substance ata 7, .In a system. for. maintaining aheated subfstanca atzsubstantially a predetermined temperature, thefcombination; orav heater, asinelethrow f-thermostatic. switch operable response to.- occurrence of said. predetermined temperature. in the. substance. and` centered for operation` about said, temperature, a secondary heat control means operatively associated `with said heater and adapted. to effect. minor changes. in the temperatureofjsaid substance. over a. given. operating cycle.. represented asfO U,. O beingv equaly to the. elapsed time. during said cycle that said substance. remains. in. excess of said. predetermined temper.ature, .U being. equalto the. elapsed time during. said; cycle. that. sai-d` substance. remains below saidpredetermined, temperature, and a princi-pal heat control means. also operatively assoeiated. with said heat-er. for .governing the thermal output thereof, saidpr-incipal control. means being' adapted to increase. the-thermal1 output. of said' heater. ataV maximum rate when the ratio OVITisfequal' to zero and to decrease said thermal output .at a. maximum rate whenY the ratio O/U approaches infinity, said principal control means also being adapted to decrease said thermalA out;- put at arate proportional to U/O Lwhen Oj/U has avalue between l .and innnity and to;A increase. said thermal' output .at a. rate proportional toY l O`/U. when. OU. has a valuebetweengzero and'. unity.

8;.. Asystemior controlling the application of heat. to anelectrolytifc substance for. the mainte.- nan-ce of a. predetermined temperaturel therein, said. system. comprising the. combinationv of means, deiiningv a. heating, chamber for the substance, ellectrodes adapted to. pass an .ele ctric. cur,- rent through the, substance. within. sai-.d heating chamber,I means. responsive to.Y the. temperature Off the.. substance within said heating, chamber. meansincluding. an adjustably positionablemem.-

ber. for; varying the effective resistance; to. they passage. of. currentV through.. the. substance. Within said.. heating; chamber-, meansgoverned by said' temperature responsive.- means. for shifting. said a'diustably positionablemember to; vary. progresssively the. power input to the` substance within said heating; chambenand additional. means. also governed by.l said temperature responsive means for effecting immediatefbutminor. alterationsin the power input to the substance withinv said heating chamber Without alteringy the position of said adjustably positionable member..

9.. A.'- system. for controlling the application. of heat` to an electrolytic substance for. the maintenance of a predetermined. temperature therein., said system. comprising. the combination of means den-ning a heating chamberfor the.. sub.- stance, electrodes. adaptedA to,` passr an electric currenttthrough. the. substance within said. heat.- ing. chamber, a. thermal switch responsive. to they temperaturer of, the substance. leaving. .said heating chamber, a. dielectricv shield adjustably positionable relative to,v said, electrodes. and adapted to vary the. eiective resistance tothe passage of. current. through. the. substance, within said heating chamber, electrical control means governed by said. thermal` switchfor. shifting. said dielectric. shield. to vary progressively the. power input to the, substance. within said heating chamber, and. additional. electrical, control means.. also governed by saidthermal switch for effecting a relatively small change in the potential applied to said electrodes accompanied by a corresponding change. in the power input to. the. substance. within said heating: chamber, said` relatively small change taking place. without. altering. the; position. of. said. adjuetably 12ositionable member.

10. A control system. for raising; the temperature of an electrolytic fluid. to a predetermined level through. the controlled application of' heat thereto, said system comprising the combination of. a. heater having fluid inlet and discharge connections, spaced electrodes adapted to pass an electric. current through the fluid within said heater, a thermal' switch responsive to the final temperature of the uid passing through. said heater, a principal heat control' means for i'nstituting a continuing, progressive and corrective change in the eiective resistance between said electrodes to correspondingly change the rate ofv heat application of. said heater within periods following the response of; said thermal switch to a final fluid temperature departing4 from the predetermined l'eveL. said principal heat control means beingv governed by said thermal. switch, and a secondary heat control means' also govacapara in the potential applied to said electrodes following response of said thermal switch to a iinal iiuid temperature departing from the predetermined level, the corrective change in applied potential being accompanied by a corresponding change in the rate of heat application.

1'1. A control system for increasing the temperature of an electrolytic fluid to a predetermined Value through the controlled application of heat thereto, said system comprising the combination of means defining a heating chamber for the fluid, spaced electrodes for passing an electric current through the iluid within the heating chamber, a thermostatic device responsive to the final temperature of the fluid discharged from said heating chamber, a principal heat control means governed by said thermostatic device and adapted for correctively moditracting a given increment from the potential applied to said electrodes by cutting a relatively small reactance into or out of their circuit. said 'secondary heat control means being adapted to correctivelv modify the rate of heat energy in-,

put to the iiuid within said heating chamber immediately upon departure of the final iiuid temperature from said predetermined value.

12. A control system for raising an electrolytic fluid to a predetermined temperature through the controlled application of heat thereto, said system comprising the combination of a heater, spaced electrodes for passing an electric current through the iiuid in said heater. means responsive to the temperature of the uid discharged from said heater, a principal heat control means for instituting a continuing and progressive decrease in the effective resistance of the fluid between said electrodes within periods following the response of said lemoerature responsive means to a temperature below a predetermined level and for also instituting a continuing and progressive increase in the effective resistance of the uid between said electrodes within periods following the response of said temperature responsive means to a temperature above the predetermined level, said principal heat control means being governed by said temperature responsive means, and a secondary heat control means also governed by said temperature responsive means and adapted for instituting an immediate, total and limited increase in the potential applied to said electrodes following response of said temperature responsive device to a temperature below the predetermined level and to institute an immediate, total and limited decrease in the potential applied to said electrodes following response of said temperature responsive device to a temperature above the predetermined level.

13. A system for maintaining a heated electrolytic fluid at a predetermined temperature and comprising the combination of a heater havving a iluid inlet and a fluid dischargel electrodes for heating the fluid within said heater by passh ing an electric current therethrough, a bimetallic thermal switch disposed for response to thev 14 temperature of the fluid discharged from said heater, said switch being adapted to open when the fluid temperature departs in one direction from the predetermined temperature and to close when the fluid temperature departs in the opposite direction, a control relay operable in Aresponse to actuation of said bimetallic switch,

a relatively small electrical resistor series-connected in the power supply circuit of said electrodes, a by-pass switch operable by said control relay for alternatively cutting said resistor into the electrical circuit or shunting said resistor out of said circuit to effect a relatively slight change in the potential applied to said electrodes, a dielectric shield disposed for movement relative to said electrodes to vary the effective electrical resistance therebetween, a reversible motor for adjustably positioning said shield, and a reversing switch operable by said control relay for driving said motor in alternative directions depending upon the condition of said bimetallic switch.

14. For use in a temperature control system having principal and secondary heat control means both governed by a thermostati'c device so as to maintain the temperature of an electrolytic uid at a predetermined level, a heater comprising the combination of means defining a heating chamber for the electrolyte, spaced electrodes adapted to pass current through the electrolyte Within said heating chamber, and an adjustably positionable member subject to automatic actuation by the principal heat control means but not said secondary heat control means for varying the power input to said electrolyte by varying the effective resistance to the flow of current between said electrodes.

15. For use in a temperature control system having a principal and a secondary heat control means both governed by a thermal switch so as to maintain the temperature of an electrolytic uid at a predetermined level, a heater comprising, in combination, a hollow body having a fluid heating chamber communicating with an inlet and an outlet, electrodes mounted in spaced relation on said body and adapted to pass current through the fluid as the latter ows through said heating chamber, a dielectric shield or paddle-like form housed within said body and interposed between said electrodes, a supporting stem fixed to said dielectric shield and journaled in a panel of said body, a reversible motor by means of an electric current, a single throw thermostatic means adapted to respond upon occurrence of the predetermined temperature level in said substance, a principal control means governed by said temperature responsive means for instituting a continuing and progressive decrease in the potential applied to said terminalsin Aresponse to a temperature labove saidpredetermined level and a like increase in potential in response to a temperature below the predeterrectively modify toithe-uidW-it l den ure; of; i said. predetermined value.

mined level.. and` a, secondary.l control... :meansv` also sdverned: by said temperature responsive means,

saisiL secondary con-troll. means being, adapted t effect immediately an increment, inthe; poten.- tial. applied. on said tel'm-inalswhen; said-tempera.-

ture:A responsive.l means. reacts. toA a. temperature.

decrease. belowl said predetermined. level. and to eff-.ect immediately,Y a. decrement. in said. potential when said@ temperature responsive. means.. reacts to a temperature. rise. above. said predetermined level..

17..A A system.. iorA controlling thev application of) heatv to. a, substance for the maintenance. ol" a. predetermined temperature level thereinv and comprising. in..r combination. a. heater adapted. to `hold said substance. spaced. terminals for. sub- Ilooting.. the. substance. in. the, heater to. heating means. of. an electric current. axsipsle.j throw 'thermostatic device responsive to. the predeterinned temperature level in said siibstance.,. a

principal. heatj control. means. governed. by said Ytherrnostatic device for instituting. a continuing jcontrolmeans also governedy by said thermostatic devicec, said secondary heaty control means. being yadapted, to eliect; immediately an. increment. in the potential applied on. said terminals when said. thermostatic device responds to a temperature. decrease below. saidv predetermined level and to. efect immediately a decrement in said potential when said temperature responsive means responds to a` temperature rise. above said pre- 'det'ermined' level, said principal and said secondary heat. control means. being so constructed that. within a predetermined time. interval m- Inediately following response to a change intemperature the. change in rate of' heat application elected. by the secondary controlv means will be greater than that effected by the principal control means and in any such period greater than said. time. interval the effect of' the principal control, means upon the .rate of'heaty application will. be. greater than that of the secondary control, means...

18. A. control system. for increasing thel temperature of an. electrolytic fluid to a predetermined value. through the controlled application of` heat. thereto, said system. comprising the. coinbinaton. of a... heater. for thel uuid, spaced eleotrades.` for passing an electric. current through the. fluid in.; said heater, a single bimetallic therinostatic dev-ice. responsive.r tothe naal temperatur-.coi the duid cischraed. from. said heater,

a.. principal heat control. means governed. by hsaid thermostatic device and adapted for correotively modifying the. rate oi. heat energy input to the huidl within said heater by instituting. ay continuous. and progressive. change. in the potential al?- i plied to, saidv electrodes upon departure of the final fluid temperature from the predetermined i value, and4 a.. secondary heat, control means also governed by said thermostatio device for alternatively' modifying by a. relatively small increment. the,y potential applied to said electrodes l through ther expedient of effecting a relatively l small-change in. their circuit reactance, said secandai-v heat control means beine:y adapted. to corthc; rate.;- oi. heatfcnergy input said. heater immediately upon final iiuidf 'temperature from 1,9. A system for maintaining. a heatedelectrolytic fluid at a` predetermined temperature, and comprising the combination. ofV a heater having a uid inlet and a fluid. discharge, electrodes for heating. the fluid. within4 saidv heater. by passing an electric currenttherethrough, a single throw thermal switch. disposed for. response to. the temperature of the fluidA discharged from said. heater, said. switch being adapted. toopen when. the fluid temperature departs in one direction from the predetermined temperature and to close when the fluid temperaturedepartsin the opposite di.- rection, a. controlrelay operable in. response to actuation of said` thermal. switch, a saturable reactor; series-connected in the power. supply circuit.v ot said electrodes, a control circuit. for the saturating winding.. of said reactor, means. governed by, said thermal switch and said control relay. for;l eiecting relatively slightandimmediate changes in the. current flow ot said control. crcuit,` and means also governed by said thermal switch. and; said control relay for eiecting gradual and progressive changes in the current now of said control circuit, all of. said changes. in the current of said control circuit serving to vary the potential appliedy to said. electrodes.

20. In a temperaturel control system. of the character set forth, the combination comprising a heater having a fluid inlet and a. iiuid discharge, terminaison said heater for heating the iiuid therein by means. of an. electric current, a single bimetallic switch responsive to the temperature ofthe uid discharged from saidheater, a. control relay operable in response'to actuation of said. bimetallic switch, a saturable reactor series-connected in the power supply circuit of saidV terminals, acontrol circuit for the saturating winding ofv said reactor, a relatively small resistor series-connected. in said` control circuit, a by-pass, switch operable by said, control relay for alternatively cutting. said. resistor into or shunting` said resistor out of said control circuit to effect corresponding variations in the saturation of said reactor and in the potential applied to said tellrlinalsyV ar rheostat in said control circuit adapted to, produce progressive changes in the current ow therein with, corresponding changes lapse of a predetermined time interval.

21.- A. temperature control system for increasingl the temperature of a moving stream` ofv :duid to a predetermined value through the controlled 'applicationv of heat thereto, saidV system comprising the combination of means defining a heating chamber for the fluid, mainy and auxiliary heating elements within said heating chamber, a thermostatio device responsive to the final temperature of the fluid discharged from said heating. Chambon, a. principal heat control means governed by saidY thermostatic means and adapted for correctively modifying the thermal output rate 0f said main heating glement continuously andY progressively upon departure of the uid temperature from the predetermined, value, and a. secondary heat. control means also. governed. by said tberrnostatic. device and. adapted. to. correctivcly modify` the thermal output. rate of said auxiliary heating element-immediately upon departure of the final fluid temperature from said predetermined value.

22. A temperature control system for controlling the application of heat to a fluid substance for the maintenance of a predetermined teiru perature therein, said system comprising the combination of means dening a heatingr chamber for the fluid, main and auxiliary heating elements within said heating chamber, a thermal switch responsive to the temperature of the iiuid discharged from said heating chamber, a control relay operable in response to actuation of said thermal switch, valve means operable by said control relay for correctively modifying the thermal output rate of said auxiliary heating element immediately upon departure of the final iiuid temperature from the predetermined ternperature, a reversible motor operable by said control relay through a, reversing switch, and addi- REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,960,162 Moorshead May 22, 1934 2,052,396 Getchell Aug. 25, 1936 2,100,327 Getchell Nov. 30, 1937 2,190,232 Fry Feb. 13, 1940 2,311,118 Matthews et al. Feb. 16, 1943 

